I'm still a fan of Firefox over IE. I'm thinking about giving IE 10 a try, the latest that I've used is IE 9, and I wasn't really impressed. It's hard for me to give up on Chrome though..
Firefox Memory usage has improved a lot in the newest version which is 23, and does not use as much Memory where it slows down my computer like in older versions where it uses more Memory.
I also think I would use Mozilla Firefox since Opera's new user interface is too similar to Google Chrome now, and Google Chrome user interface feels like it has fewer features since there is not a bookmark bar, menu bar, status bar, and I lke the download manager in Firefox better.
IE also has too many options/settings which are kind of confusing. Although, IE 10-11 seems faster these days.
Firefox add-ons do not occupy much memory like it used to though and of course I would pick Firefox over IE because of the size. I also prefer free and open-source software to these commercialized wares.
I haven't had very good support with Microsoft support forums, and sometimes end up just using a search engine like Google, DuckDuckGo, etc to look for a blog post with someone with a similar Windows or IE problem for an answer on how to fix it.
With Firefox running on Ubuntu Linux or Linux Mint, I had very few problems with it since Linux runs programs better than Windows, and I notice Firefox feels a lot faster on Linux Mint and Ubuntu than Windows on a slower computer with an older Intel CPU, and 1GB of RAM.
There is also no IE for Android, so you can't sync your bookmarks, open tabs, passwords and user nemae, and other account info like Firefox where you can sync your Firefox bookmarks, etc with your Android phone/tablet with Firefox, other computers with Firefox, etc.
Firefox is also available for most versions of Windows and Linux instead of IE forcing you to upgrade to Windows 7-8 to use IE 10, or upgrade to Vista to use IE 9.